allow for custom lights from users to be saved

[kfarr]

As a user I want to create a scene with a spotlight. I want the built-in lights from the scene to remain the same and I want to add an additional spotlight. When I save the file I expect the spotlight to come back.

Issue:

• right now the spotlight is not saved, nothing under environment is saved on purpose as part of https://github.com/3DStreet/3dstreet/issues/292 "do not save any children of environment component such as lights or sky..."

Questions:

• how can we differentiate between user lights that are meant to be saved and auto generated lights that are not to be saved?
• what if a user clones a built-in light and changes its values -- will that light be saved?
• are there other "environment" entities that a user may want to edit besides lights?

[kfarr]

workaround:

• create a mixin that is a light an add entity with that mixin in scene?
• create a light entity from scratch (maybe even in the json) and add to the scene?